The long-term goal of our research is to understand the molecular mechanisms that control centrosome biogenesis. Centrosomes are microtubule-nucleating organelles that play a central role in most microtubule- related functions, including cell motility, intracellular transport and chromosome segregation. In addition, the centrosome forms the basis of the basal body, which is required for the formation of cilia and flagella, and i therefore crucial for cells to both sense their environment and transduce signals. Centrosomes are present as a single copy at the beginning of the cycle and duplicate once during S phase to ensure only two copies are present to organize the poles of the mitotic spindle. Abnormalities in centrosome number are commonly observed in human cancer cells, where extra centrosomes lead to chromosome segregation errors that are thought to drive tumor formation. Understanding the mechanism by which cells achieve the once per cycle duplication of the centrosome is therefore an important fundamental question of considerable relevance to human health. Polo-like kinase 4 (Plk4) has emerged as the central, dose-dependent regulator of centrosome duplication. Suppressing Plk4 inhibits centrosome duplication, while Plk4 overexpression leads to the production of too many centrosomes. However, we understand little about how this kinase functions; and in particular, the critical Plk4 targets that control centrosome biogenesis remain to be identified. Our proposed research seeks to establish the mechanisms through which Plk4 orchestrates and coordinates centrosome biogenesis. Previous efforts to study Plk4 have been hampered because tools to specifically and rapidly manipulate Plk4 function have not been available. In this application we have overcome this limitation by developing two complementary methodologies that allow us to regulate Plk4 levels and activity in living cells. Using these tools we will establish the direct effect of altering Plk4 levels/actiity and distinguish between kinase-dependent and scaffolding functions of Plk4 in centrosome biogenesis. In Aim 1, we will study the effect of rapid loss/inhibition of Plk4 on cell growth and centrosome structure. In our preliminary data we have identified a highly conserved centrosome protein as a key Plk4 substrate required for centrosome duplication. In Aim 2, we propose to characterize how Plk4-mediated phosphorylation of this substrate contributes to centrosome assembly. These studies are relevant for understanding the normal regulation of centrosome assembly and for furthering ongoing efforts to target Plk4 in cancer therapy.